Energy consumption worldwide is generally increasing, and conventional hydrocarbon resources are being consumed. In an attempt to meet demand, the exploitation of unconventional resources may be desired. For example, highly viscous hydrocarbon resources, such as heavy oils, may be trapped in tar sands where their viscous nature does not permit conventional oil well production. Estimates are that trillions of barrels of oil reserves may be found in such tar sand formations.
In some instances these tar sand deposits are currently extracted via open-pit mining. Another approach for in situ extraction for deeper deposits is known as Steam-Assisted Gravity Drainage (SAGD). The heavy oil is immobile at reservoir temperatures and therefore the oil is typically heated in-situ to reduce its viscosity and mobilize the oil flow. In SAGD, pairs of injector and producer wells are formed to be laterally extending in the ground. Each pair of injector/producer wells includes a lower producer well and an upper injector well. The injector/producer wells are typically located in the payzone of the subterranean formation between an underburden layer and an overburden layer.
The upper injector well is used to typically inject steam, and the lower producer well collects the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam. The injected steam forms a steam chamber that expands vertically and horizontally in the formation. The heat from the steam reduces the viscosity of the heavy crude oil or bitumen which allows it to flow down into the lower producer well where it is collected and recovered. The steam and gases rise due to their lower density so that steam is not produced at the lower producer well and steam trap control is used to the same affect. Gases, such as methane, carbon dioxide, and hydrogen sulfide, for example, may tend to rise in the steam chamber and fill the void space left by the oil defining an insulating layer above the steam. Oil and water flow is by gravity driven drainage, into the lower producer well.
Operating the injection and production wells at approximately reservoir pressure may address the instability problems that adversely affect high-pressure steam processes. SAGD may produce a smooth, even production that can be as high as 70% to 80% of the original oil in place (OOIP) in suitable reservoirs. The SAGD process may be relatively sensitive to shale streaks and other vertical barriers since, as the rock is heated, differential thermal expansion causes fractures in it, allowing steam and fluids to flow through. SAGD may be twice as efficient as the older cyclic steam stimulation (CSS) process.
Many countries in the world have large deposits of oil sands, including the United States, Russia, and various countries in the Middle East. Oil sands may represent as much as two-thirds of the world's total petroleum resource, with at least 1.7 trillion barrels in the Canadian Athabasca Oil Sands, for example. At the present time, only Canada has a large-scale commercial oil sands industry, though a small amount of oil from oil sands is also produced in Venezuela. Because of increasing oil sands production, Canada has become the largest single supplier of oil and products to the United States. Oil sands now are the source of almost half of Canada's oil production, although due to the 2008 economic downturn work on new projects has been deferred, while Venezuelan production has been declining in recent years. Oil is not yet produced from oil sands on a significant level in other countries.
Unfortunately, long production times to extract oil using SAGD may lead to significant heat loss to the adjacent soil, excessive consumption of steam, and a high cost for recovery. Moreover, there may be an insufficient amount of caprock to contain the steam, and surface water resources may be limited. It thus may be desirable to use conducted heating initially to soften the subterranean formation to establish convective flow to convey the steam. However, conducted heating is relatively slow and unreliable. Also, many SAGD wells fail to start, and SAGD may be impractical in permafrost areas due to melting at the surface.
Radio frequency heating is one approach for enhanced oil recovery (EOR). In radio frequency heating, electric fields and magnetic fields may be applied to a subterranean formation using an underground antenna. Radio frequency heating has the advantage of increased speed compared to steam.
U.S. Published Patent Application No. 2010/0078163 to Banerjee et al. discloses a hydrocarbon recovery process whereby three wells are provided: an uppermost well used to inject water, a middle well used to introduce microwaves into the reservoir, and a lowermost well for production. A microwave generator generates microwaves which are directed into a zone above the middle well through a series of waveguides. The frequency of the microwaves is at a frequency substantially equivalent to the resonant frequency of the water so that the water is heated.
Along these lines, U.S. Published Application No. 2010/0294489 to Dreher, Jr. et al. discloses using microwaves to provide heating. An activator is injected below the surface and is heated by the microwaves, and the activator then heats the heavy oil in the production well. U.S. Published Application No. 2010/0294489 to Wheeler et al. discloses a similar approach.
U.S. Pat. No. 5,046,559 to Glandt discloses a method for producing oil from tar sands by electrically preheating paths of increased injectivity between an injector well and a pair of producer wells arranged in a triangular pattern. The paths of increased injectivity are then steam flooded to produce the hydrocarbon resources.
Unfortunately, SAGD may not efficiently permit recovery of the hydrocarbon resources in that SAGD may have increased capital and energy costs, for example, as disclosed in U.S. Patent Application Publication No. 2010/0276148 to Wylie et al. Wylie et al. discloses combusting a fuel mixture so that combustion gases with relatively high levels of carbon dioxide, steam, and/or hot water are used to improve recovery of heavy hydrocarbons. In particular, a gas, fluid water, and carbon dioxide are delivered to the heavy hydrocarbon material. The gas may be heated by microwave RF heating. Still, further efficiency in hydrocarbon recovery may be desired.